1. Field of the Invention
The present invention relates to rear derailleurs for bicycles. More particularly, the invention relates to a rear derailleur including a mounting member having a coupling device for connection to a bicycle frame, a bracket member supported by the mounting member to be pivotable about a first axis extending substantially parallel to a hub axis, a support member disposed forwardly of the bracket member for supporting a chain guide wheel and a chain tension wheel to be pivotable about a second axis substantially parallel to the first axis, a pair of pivot links arranged substantially parallel to a first phantom line extending through the first axis and second axis as seen in a direction along the hub shaft for interconnecting the bracket member and support member, a first tension spring for biasing the bracket member to pivot relative to the bicycle frame, thereby to apply a tension to a chain, and a second tension spring for biasing the guide wheel and tension wheel to pivot relative to the support member, thereby to apply a tension to the chain.
2. Description of the Related Art
The bicycle rear derailleur of the type noted above has a relatively simple construction with regard to the mounting member and support member in that the first phantom line and pivot links extend substantially parallel to each other as seen in the direction along the hub shaft. Where the derailleur is the type attached to a bicycle frame having a derailleur mounting extension formed at a rear fork end of the bicycle frame, the position of the derailleur relative to the hub axis is determined by the construction of the derailleur mounting extension. The pivot links are required to have a length for causing the guide wheel to move sideways through a predetermined stroke, thereby changing the chain from one gear to another as desired. Consequently, with a shifting operation, the guide wheel is sideways movable in a region forwardly of and relatively remote from the gears.
When the rear derailleur is attached to the bicycle frame with the chain engaged with a 13-tooth gear of the rear gear assembly, a phantom line extending through rotational axes of the guide wheel and tension wheel may be adjusted to be vertical. Then the derailleur may be in a desired tension condition having an appropriate chain tension, regardless of a speed to which the derailleur is shifted. In this assembled posture, however, a phantom line extending through the hub axis and parallel to a pivotal axis about which an outer one of the pivot links is pivotable relative to the bracket member, conventionally, coincides with this pivotal axis as seen in the direction along the hub axis, or, as shown in FIG. 9, extends through a position rearwardly of the pivotal axis, with the guide wheel disposed forwardly of and relatively remote from the gears.
A derailleur of the type attached to a bicycle frame with no derailleur mounting extension provided for the rear fork end is connected to the rear fork end through a bracket. In this case, because of the shape and size of a mounting member, the positional relationship between derailleur and hub axis is substantially the same as that between derailleur and hub axis where the derailleur is the type directly attached to the derailleur mounting extension. That is, the mounting member has such a construction that the former derailleur is in substantially the same positional relationship to the bicycle frame to which the derailleur is attached as the derailleur directly attached to the bicycle frame. Thus, as disclosed in Japanese Patent Publication No. 47-27656, for example, in the derailleur of the type connected to the bicycle frame through a bracket has, as in the directly connected type derailleur, the phantom line extending through the hub axis and parallel to the pivotal axis about which the outer pivot link is pivotable relative to the bracket member coincides with this pivotal axis as seen in the direction along the hub axis, or extends through a position rearwardly of the pivotal axis. As a result, the guide wheel is disposed forwardly of and relatively remote from the gears.
Thus, in the above assembled posture, the conventional derailleurs have the guide wheel located relatively far apart in a forward direction of the bicycle from the gears. To change the chain from one gear to another, the guide wheel moves the chain sideways in a lateral region relatively remote from the gears. Consequently, the guide wheel fails to apply chain disengaging and engaging forces efficiently. Such a shifting operation does not achieve high efficiency, with a relatively long time consumed until completion of the operation.